This invention relates to lightweight paper coatings having high opacity and visual gloss. More particularly, it relates to lightweight coatings containing discrete plastic, polymeric particles having diameters in the range from about 0.3 to about 0.8 micron.
Most paper surfaces require a coating in order to have good printing qualities and sufficient opacity or hiding power. Conventional paper coatings contain inorganic pigments such as kaolin clay or titanium dioxide to give the coated substrate the opacity required. These inorganic pigments substantially increase the coating weight whicn in turn increase the cost of mailing substrates so coated.
Paper coatings which contain bubbles as substitutes for inorganic pigments have been described in U.S. Pat. No. 3,108,009. These coatings are prepared by dispersing a high boiling immiscible liquid in the binder media and then evaporating the dispersed liquid after the coating has been applied to the substrate. It is desirable in the exercise of this method to recapture the evaporated liquid in order to reduce the costs of the coating. Also, according to this method it is preferable to apply a layer of varnish to the coating containing the air spaces to make the surface more suitable for printing.
Paper containing expanded microspheres of plastics has been described in U.S. Pat. No. 3,293,144, but these expanded microspheres are considerably larger than a wavelength of visual light. Generally this kind of paper is used primarily in areas where a high degree of opacity is not required.